Myocardial ischemia results from insufficient blood flow to the heart muscle. Ischemia may occur chronically, and to varying degrees, due to coronary artery disease (CAD) or acutely due to sudden increased demand, embolism or vasospasm. Ischemia can lead to angina and eventually to myocardial infarction—permanent damage to the heart muscle. Moreover, both ischemia and infarction can trigger fatal arrhythmias.
Ischemia can be detected by electrocardiographic changes. Conventional detection is through ST segment elevation shown in surface ECG. Detection through surface ECG is done only briefly and infrequently in the clinic or through the use of a Holter monitor. Only those ischemic events which happen to occur, or which may be provoked by stress tests during monitoring are detected.
A long-term record of ischemia burden obtained through continuous monitoring would be very useful as an adjunct to current methods of ischemia detection and diagnosis. Such a record may reveal infrequent or unprovokable ischemia perhaps associated with nascent CAD, vasospasm or embolism. Such a record could reveal trends in the progression or regression of CAD. It could also be used to gauge the efficacy of, and/or patient compliance with, a course medication.
Implantable medical devices (IMDs), such as pacemakers and implantable cardiac defibrillators (ICDs), offer the ideal platform for ischemia burden monitoring. IMDs can constantly monitor the electrophysiological conditions of patients and detect the onset and/or the burden of ischemia. Prior patents such as U.S. Pat. No. 6,108,577 issued to Michael Benser or U.S. Pat. No. 6,609,023 issued to Fischell et al. describe the detection of ischemia based on ST level change detected from EGMs of implanted lead electrodes.
The capability to detect ischemia may have other applications in IMDs. Because myocardial perfusion occurs during diastole, lower heart rates are conducive to better perfusion. Therefore, an IMD should avoid pacing at high rates if ischemia is detected. An IMD may perhaps even force a ventricular rate lower than the sinus rate through special pacing techniques such as the one described in U.S. Pat. No. 6,377,852 by Bomzin et al. IMDs may also alert patients of silent (asymptomatic) ischemic events so that they may take appropriate action such as taking medication, ceasing exertion, lying down etc. IMDs may also release thrombolytic or antithrombotic medication upon the detection of ischemia.